Chemical improvement, the effect of which is generally evaluated by swell-shrink index, is a common method to treat expansive soil. In order to establish the relationship between unsaturated soil theory and engineering problem, the soil-water characteristic curve is used to evaluate the improvement effects on expansive soil. Two modifiers, lime and fly ash with four various amounts, were added into the Baise expansive soil to conduct the free expansion ratio test. The tests of dewetting soil-water characteristic curve at the range of 5～1000KPa were conducted by pressure plate apparatus. The expansibility and soil-water characteristic curve of the improved expansive soils are obtained. Based on the theory of unsaturated soil, the SWCC fitted by VG model for two modifiers with different mixing amounts are analyzed. It can be found that four indexes, saturated gravity water content, air entry value, residual water content and curve slope, can be used to evaluate the chemical improvement effects on expansive soil. The improvement effects can be determined only if three indexes met the requirements, but the saturated gravity water content is indispensable.
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